Quinacridones are known compounds which are used as pigments. In practice, stringent requirements are placed on their fastness and color properties. On the industrial scale they are prepared by oxidizing dihydroquinacridones in an alkaline medium in the presence of solvents and then dry- or wet-grinding the resulting coarsely crystalline crude pigments, or by ring closure of 2,5-dianilinoterephthalic acid in polyphosphoric acid or polyphosphoric ester and then phase-converting and finishing the resulting finely divided crude pigments with organic solvents.
The preparation of quinacridone mixed crystal pigments is described in the following patent documents.
U.S. Pat. No. 4,099,980 describes the preparation of quinacridone mixed crystal pigments, consisting of 85-99% unsubstituted quinacridone and 1-15% 4,11-dichloroquinacridone, which are in the .gamma. phase of the unsubstituted quinacridone.
U.S. Pat. No. 3,160,510 describes the preparation of quinacridone mixed crystal pigments by dry-milling the crude pigment mixtures with salt and then solvent-treating the isolated ground materials or by precipitating the pigment mixtures with sulfuric acid and then solvent-treating the dried finely divided crude pigments.
For unsubstituted quinacridone of the .gamma. phase, four phases are described.
The .gamma.I phase is described in U.S. Pat. No. 3,074,950 and in EP-A 0 267 877. In the X-ray spectrum, at twice the Bragg angle 2.theta., it shows three strong lines at 6.6.degree., 13.9.degree. and 26.5.degree., three moderate lines at 13.2.degree., 13.5.degree. and 23.8.degree., and four weak lines at 17.1.degree., 20.5.degree., 25.2.degree. and 28.6.degree..
The .gamma.II phase is described in U.S. Pat. No. 2,844,484, in EP-A 0 267 877 and in DE-C 1 184 881. In the X-ray spectrum, at twice the Bragg angle 2.theta., it shows three strong lines at 6.6.degree., 13.9.degree. and 26.3.degree., five moderate lines at 13.2.degree., 13.4.degree. and 23.6.degree., 25.2.degree. and 28.3.degree. and two weak lines at 17.1.degree. and 20.4.degree..
The .gamma.III phase is described In EP-A 0 530 142. In the X-ray spectrum, at twice the Bragg angle 2.theta., it shows four strong lines at 6.7.degree., 13.3.degree., 14.0.degree. and 26.6.degree., one moderate line at 13.6.degree., and seven weak lines at 17.2.degree., 20.6.degree., 21.9.degree., 24.0.degree., 25.3.degree., 28.1.degree. and 28.8.degree..
The .gamma.IV phase is described in Japanese Laid-Open Specification JP-A 53-39324. In the X-ray spectrum, at twice the Bragg angle 2.theta., it shows three strong lines at 6.2.degree., 13.6.degree. and 26.5.degree., three moderate lines at 12.5.degree., 25.8.degree. and 27.7.degree., 2.theta. and three weak lines at 16.5.degree., 20.5.degree. and 24.0.degree.. The moderate lines at 25.8.degree. and 27.70.degree.2.theta. are to be attributed to small amounts of .alpha. phase.